Forty-nine mutilated bodies have been found dumped by a roadside near the city of Monterrey in northern Mexico.
Security officials said the 43 men and six women had been decapitated and had their hands cut off, making identification difficult, the BBC reports.
They blamed the killings on a conflict between rival drugs gangs - a note left with the bodies said they had been killed by the Zetas cartel.
It is the latest in a series of recent massacres in northern Mexico.
The Zetas have been fighting the Gulf and Sinaloa cartels for control of smuggling routes into the United States.
The bodies were found at 4am local time on Sunday in Cadereyta municipality on the road from Monterrey to Reynosa on the American border.
Security officials said the bodies appeared to have been killed at another location up to two days ago and dumped from a truck.
The grim find comes just days after police discovered the dismembered, decapitated bodies of 18 people in two abandoned vehicles in western Mexico.
Earlier in May 23 dead bodies - 14 of them decapitated - were found in the border city of Nuevo Laredo, also in Nuevo Leon state.
About 50,000 people have died in drug-related violence in Mexico since 2006, when President Felipe Calderon deployed the army to combat the cartels.